Dielectric microspheres or planar waveguide ring- or disk-shaped resonators have attracted increasing attention in sensing applications, including biosensing. The size of these types of resonators typically ranges from approximately 20 μm to a few millimeters for microspheres and from 5 μm to a few tens of microns for ring- or disk-shaped resonators. Such microspheres and ring- or disk-shaped resonators are often referred to as microresonators. In the most common configuration in microresonator-based biosensors, a microresonator is placed in a close proximity to an optical waveguide such as optical fiber whose geometry has been specifically tailored, for example tapered or etched to a size of 1-5 μm.
The tapering modifications to the waveguide result in there being a substantial optical field outside the waveguide, and thus light can couple into the microresonator and excite its eigenmodes, often referred to as whispering gallery modes (WGMs). When microresonators made with low loss materials and with high surface quality are used, the propagation loss of light propagating in WGMs is very low, and extremely high quality factors, also known as Q-factors, can be achieved: values as high as 109 are achievable. Due to the high Q-factor, the light can circulate inside the microsphere resonator for a very long time, thus leading to a very large field enhancement in the cavity mode, and a very long effective light path. This makes such devices useful for non-linear optical and sensing applications. In sensing applications, the samples to be sensed are placed on the sphere's surface, where they interact with the evanescent portion of the WGM available outside the microresonator. Due to the enhanced field and the increased interaction length between the light and samples, the microresonator-based optical sensors feature high sensitivity and/or a low detection limit.